1. Field of the Invention
The present invention relates to an image forming apparatus provided with a sheet refeeding unit, wherein copy sheets ejected from an image forming section are collected to be fed back to the image forming section for duplex or composite copying.
2. Description of prior Art
A conventional sheet refeeding apparatus is so constructed that copy sheets which obtained an image are collected on a horizontally or vertically disposed tray in the stack condition, and thereafter the sheets are fed out thereof one after another by a roller such as a rubber roller. However, copy sheets are apt to curl because of the heat of a fixing device in a copying machine, and accordingly it is difficult to guide and place such copy sheets onto the tray. Also, the conditions of copy sheets and the rubber roller are changeable depending on temperature and humidity, so that it is difficult to feed collected copy sheets exactly one by one in any condition. Therefore, there have been a fear of occurring such a trouble as a paper jam and double feeding in such a sheet refeeding apparatus.
Under the circumstances, a roll-up type sheet refeeding apparatus which comprises a belt supported on one roll or two rolls, which is driven to wind copy sheets ejected from a copying machine on the roll so that the copy sheets may be stored between the layers of the wound belt and driven to unwind from the roll so that the copy sheets are fed out, was proposed in U.S. Pat. No. 3,862,802 (corresponding to Japanese Patent Publication No. 57-42861), Japanese Patent Laid-Open publication No. 58-188260 and Japanese Patent Laid-Open Publication No. 59-39561 (corresponding to U.S. Pat. No. 4,496,142). In this type of apparatus, the collecting and refeeding of copy sheets are surely performed. However, here is a problem that copy sheets fall into a habit of bending because of the wind on the roll together with the belt. If the direction of the bending habit is the same as that of the curvature of a photosensitive drum, a copy sheet fed back to an image transfer section from the refeeding apparatus will be hard to separate from the surface of the photosensitive drum, and therefore a paper jam may occur at the transfer section frequently.
Incidentally, sheet refeed is generally required for an operation in a duplex copying mode wherein a copy sheet is subjected to copying operations to obtain images on its both sides and an operation in a composite copying mode wherein a copy sheet is subjected to copying operations to obtain images on its one side. When a roll-up type sheet refeeding apparatus as described above is attached to a copying machine which is operative both in the duplex copying mode and in the composite copying mode, the direction of the bending habit of copy sheets will be the same as that of the curvature of the photosensitive drum in either the duplex or composite copying mode, since the direction of winding copy sheets is fixed in the both modes, and therefore the occurrence of a paper jam becomes a problem.
Also, there are other problems with regard to this type of sheet refeeding apparatus. One of the problems is how to take timing of returning the belt to the initial position where a sheet collecting operation is available. A return of the belt to the initial position in advance of a sheet collecting operation is necessary to make good use of the belt. However, it takes some time to return the belt, and if the timing of returning the belt is not taken properly, an operator may have to be wait unreasonably.
Further, another problem is how to detect collected copy sheets. For instance, even if the belt is made of transparent material so that sheets received on the belt can be detected by a sensor, the detection will be difficult since the diameter of the roll of the belt fluctuates heavily. Even if mechanical means of detecting sheets is used, the structure will be complicated for the same reason. Furthermore, it will also be a problem how to comply with such troubles as an operator misoperates the apparatus to collect sheets although the sheets are not supposed to be collected, and a sheet remains in the apparatus on starting a copying operation. Such troubles are made in the following cases.
(1) The previous operator forgot to take sheets out of the sheet refeeding apparatus.
(2) The operator selected the duplex or the composite copying mode by mistake and a copying operation of a first image has been performed in the misselected mode.
(3) A supply of power for the apparatus is discontinued during a sheet collecting operation. It is substantially impossible to take sheets out of the belt by hand in such a case.